When the San Antonio heat is in full swing, the last thing you want is warm air coming from your AC vents. If your air conditioner is running but not cooling your home, it’s more than just uncomfortable—it’s a sign something’s not working right.
At ASC Heating & Air, we know how frustrating this can be, and we’re here to help.
In this blog, we’ll cover the top five reasons your AC might be blowing warm air and what you can do to get your cool air back fast.
The 5 Main Reasons Your AC Might Blow Warm Air
1. Your Air Filter Is Dirty or Clogged
A dirty air filter blocks airflow, which means cool air can’t get through your vents. Instead, you might feel weak airflow — or worse, warm air — blowing through the house.
How to fix it:
- Turn off the system
- Remove the filter and hold it up to the light
- If light can’t pass through, replace it with a clean one
A clean filter can restore normal airflow and prevent your AC from freezing up. If warm air continues, the issue might have already affected other parts of the system.
2. Thermostat Is Set Incorrectly or Malfunctioning
If the thermostat is set to the wrong mode, your AC might be running without cooling. This can happen if the system is set to “Heat” or the fan is left on “On” instead of “Auto.”
What to check:
- Make sure the mode is set to “Cool”
- Set the fan to “Auto,” not “On”
If the settings look right but the AC is still blowing warm air, the thermostat could be misreading temperatures or not communicating with the system. You’ll need a professional to test and calibrate it.
3. Low Refrigerant or a Refrigerant Leak
Refrigerant is what allows your AC to remove heat from the air inside your home. If your system is low on refrigerant — or if there’s a leak somewhere in the line — you’ll notice warm air coming from the vents, even though everything seems to be running.
What to look for:
- Ice buildup on the refrigerant line or evaporator coil
- A hissing or bubbling sound near the outdoor unit
- AC runs for long periods but doesn’t cool the house
You can’t refill refrigerant like you would gas in a car. It requires specific tools, gauges, and refrigerant types — plus the leak has to be found and sealed first. That’s where ASC Heating & Air comes in. Our licensed technicians can inspect your system, repair the leak, and safely recharge it to the correct level, ensuring your home stays cool again.
4. Outdoor Unit Is Dirty or Blocked
Your outdoor AC unit (the condenser) is responsible for releasing heat that your indoor system removes. If it’s clogged with dirt, leaves, or grass, it can’t do its job—and instead of cool air, you’ll feel warm air inside.
Try this first:
- Turn off the system
- Clear away any debris around the unit
- Use a hose with light pressure to rinse the coils from top to bottom
If it’s been a while since your last cleaning, or if your unit is still struggling after this, it may need a deeper service. Our team at ASC can professionally clean the coils and inspect the unit for wear or damage that could be affecting performance. A clean, clear condenser makes a huge difference in cooling efficiency.
5. Electrical or Compressor Problems
Even if your thermostat is set correctly and your filter is clean, your AC can still blow warm air if the outdoor unit loses power or the compressor fails. These issues usually stop the cooling process completely, leaving only the fan running.
You might notice:
- The indoor unit is blowing, but the outdoor unit is silent
- Breakers in your panel are tripped or keep tripping
- Buzzing, clicking, or no sound at all from the outdoor unit
These types of problems often involve high-voltage parts and shouldn’t be handled on your own. ASC’s trained technicians can safely test the system, repair damaged components, or replace faulty compressors if needed. We’ll get your system back to cooling without the risk of electrical damage or personal injury.
Other Quick Things That Can Go Wrong
Not all AC problems show up in obvious ways. Even if your system is running, these hidden issues can still cause warm air, poor airflow, or even system shutdowns.
Frozen Evaporator Coils
If the coils inside your AC freeze, the system can’t absorb heat—and you’ll end up with warm or no air at all. This usually happens when there’s poor airflow due to a dirty filter or a blocked return vent.
What to do:
- Turn off the AC and let the coils thaw completely (do not chip at or knock off the ice)
- Check and replace the air filter
- Avoid running the system until airflow is restored
If the coils freeze again, there may be a deeper airflow or refrigerant issue. It’s best to have a technician take a look.
Blocked Condensate Drain Line
Your AC pulls moisture from the air while it cools. That moisture drains through a small pipe, but if it gets clogged with algae or debris, water can back up and trigger a system shut-off.
Signs of a clog:
- Water pooling around the indoor unit
- Musty smells or mold growth
- AC shuts off unexpectedly
If you spot water where it shouldn’t be, turn off the unit and call for service. Clearing the line quickly helps prevent water damage and mold buildup.
Leaky or Damaged Ductwork
If cool air is leaking into your attic, basement, or crawlspace, your rooms won’t get the comfort they need. The system will still run, but you’ll get warm air—or uneven cooling—throughout the house.
What to check:
- Certain rooms are much warmer than others
- Weak airflow even when the system’s running
- Rising energy bills without a clear reason
Sealing or repairing ductwork requires specialized tools and access. A professional can inspect your duct system and fix leaks that waste cool air.
Still Feeling Warm Air? Let’s Get It Fixed
If your AC is blowing warm air, don’t wait for the San Antonio heat to make things worse. Whether it’s a simple thermostat issue or a more serious refrigerant leak, identifying the problem early can save you time, money, and discomfort.
At ASC Heating & Air, we’re committed to keeping your home cool and comfortable with fast, reliable service you can count on. If you’re still not sure what’s causing the issue—or you’d rather have a professional take a look—give us a call at (210)761-5130. We’re here to help you stay cool, no matter how hot it gets outside.
